Just got my kit and now have scraps to try out. Using the Aliexpress leather iron at max setting. I found that a test strip of 250GSM (tube fabric) on 450GSM (floor fabric) would separate by the TPU peeling off the floor fabric (see photo)
Joining 250-250 resulted in a very strong bond that eventually tore with some TPU on each side. Joining 450-450 was tricky as the matte side seemed to stick to the iron so used parchment paper. The heavy stuff took longer to heat, but after cooling came off all on one side.
It feels like the TPU comes off more easily on the heavier stock (black 450GSM material). Does that sound right?